The present invention relates to a space diversity system for multi-direction time division multiplex (MD-TDM) terrestrial communication network, comprising a great number of scattered satellite stations and one common central station, and more particularly to a space diversity receiving system for a central station.
In a conventional MD-TDM communication system, time division multiplexed (TDM) signals are simultaneously transmitted to a plurality of satellite stations, in a continuous mode. On the other hand, each satellite station, in synchronism with a clock at the central station, transmits burst signals to the central station in a time slot which is individually assigned thereto. Burst signals from the satellite stations are regularly aligned in a time sequence at the central station, with the result that burst signals from any satellite station can be readily identified and reproduced according to the clock at the central station.
In such an MD-TDM communication system, if the transmission path between the central station and a given satellite station extends over a long distance or shows a worse propagation condition like over a sea, the radio transmission quality requires compensation preferably by the use of space diversity reception.
Meanwhile, as a space diversity system in a two-way communication between two geographically separated stations, there is available a baseband switching system in which signals received from a main and a supplementary antennas are separately demodulated. The demodulated signal having a lower bit error rate is selected. For details on this space diversity reception system, reference is made to Yamazaki, et al., "2 GHz Digital Radio-Relay System", IEEE International Conference on Communication, Vol. 1, pp. 5.5.1-5.5.5, 1979.
However, this baseband switching system has a disadvantage. If it is directly applied to the aforementioned MD-TDM communication system, it will be impossible at the central station which receives signals from a plurality of satellite stations to separately monitor and control the channel conditions for each satellite station. Accordingly, no sufficient space diversity effect can be achieved.